XRF is a reliable, sensitive and widely used technique for the detection and quantification of elements of interest in various sample types. In XRF analysis, a radioisotope source or x-ray tube generates a beam of exciting radiation which is directed onto a sample. Elements in the sample responsively emit fluorescent x-rays of characteristic energies. The emitted x-rays are sensed by a detector, which produces signals representative of the energies of the received x-rays. These signals are processed to construct a spectrum of x-ray count rate as a function of energy. The concentration of an element of interest, such as lead, may be determined from the intensity of its characteristic peak(s) in the spectrum.
One growing application of XRF analysis is the screening of consumer goods, such as toys, for the presence of lead and other hazardous elements at unacceptably high concentrations. Consumer goods are typically of heterogeneous construction, having multiple layers of material of varying thicknesses, and an instrument operator will often have little or no information regarding the specific construction of an inspected object at the time of testing. However, it may be desirable or necessary to identify the location within the inspected object of the detected element of interest e.g., whether the element is located in a coating layer or in the underlying substrate. Toys, for example, will generally have a thin layer of paint or other coating overlying a bulk material. If lead is detected in the toy, it may be important to discern whether the lead is present primarily in the paint or primarily distributed through the bulk material, since the applicable regulatory standard and optimal method for calculating the lead concentration can depend on the location of the lead.
The location of a detected element of interest may be determined by examining the intensity ratio of two characteristic x-ray lines corresponding to the element of interest (e.g., the Lα and Lβ x-rays of lead), one of which is preferentially absorbed relative to the other (see, for example, U.S. Patent Application Publication No. 2009/0067572 by Grodzins et al.) This approach, while providing satisfactory results for certain applications, may not be suitable for a broader range of elements of interest and sample matrices. For example, it may be difficult to distinguish coating lead from bulk lead in thin, low-density materials such as polyethylene. Furthermore, the intensity ratio method cannot be employed, except in special cases, to elements whose concentration is measured by the intensity of the K x-rays.
Thus, there remains a need in the art for localization technique that may be successfully and reliably employed for XRF analysis of a variety of elements of interest in disparate types of samples.